The present invention relates to a method for the preparation of a low-melting potassium fluoroaluminate, to the product obtainable according to the method of the invention, and to the use of the product as a flux for soldering light metal materials.
In hard soldering or oven soldering, metal materials, in particular aluminum, are joined at a temperature above 500.degree. C. with the aid of a molten added metal (solder). A flux is added in order to remove oxides and other disruptive covering films on the metal surface. For this purpose, in particular, fluxes composed of potassium fluoroaluminates are used. The production of potassium fluoroaluminates has been investigated from both a technical and a scientific point of view. According to the publication by I. V. Tananaev and M. A. Nekhamkina in Izvest. Sektora Fiz.-Khim-Anal., Akad. Nauk S.S.S.R. 20, pages 227 to 237 (1950), potassium fluoroaluminates may, for example, be prepared from (highly diluted) solutions of potassium fluoride and aluminum fluoride. At a molar ratio of KF to AlF.sub.3 of 2:1 to 10:1, substantially the hydrate of dipotassium pentafluoroaluminate (K.sub.2 AlF.sub.5.H.sub.2 O) is apparently produced. The melting point of this product according to the Russian publication is 855.degree. C. Although monopotassium tetrafluoroaluminate and tripotassium hexafluoroaluminate exist, they are not produced according to Tananaev et al., at least in the quoted range for the molar ratio of KF to AlF.sub.3 of 1:1 to 10:1. A potassium fluoroaluminate having such a high melting point would be unsuitable, for example, as a soldering flux for soldering aluminum.
Willenberg et al., U.S. Pat. No. 4,428,920, discloses a method for the preparation of potassium tetrafluoroaluminate having a melting point below 575.degree. C. In this method, fluoroaluminium acid is reacted with potassium hydroxide. It is noted that no satisfactory product is obtained when using equimolar quantities of the reactants, and it is therefore proposed to introduce a less than stoichiometric amount of potassium into the reaction mixture. The lowest melting point of the products prepared is 570.degree. C.
According to Shimizu et al, U.S. Pat. No. 5,171,377, a melting point of 560.degree. C., as observed in potassium fluoroaluminates, is still regarded as being too high. The lowering of the melting point, as is achieved by admixing aluminum ammonium fluoride or by using mixtures of caesium fluoroaluminate and aluminum fluoride, is regarded as having drawbacks.
Kawase, U.S. Pat. No. 4,579,605, discloses a flux for soldering aluminum which comprises a mixture of monopotassium tetrafluoroaluminate and dipotassium pentafluoroaluminate or the hydrate thereof. Such mixtures are said to melt at a temperature of 555.degree. C. to 574.degree. C. They are produced, if reaction mixtures are produced in which, as an indispensable prerequisite, the ratio of Al:F:K lies in the range from 1:4 to 4.5:1 to 1.5.